Abstract

The present study is the first to examine the effect of high altitude acclimatisation and re-exposure on the responses of cerebral blood flow and ventilation to CO2. We also compared the steady-state estimates of these parameters during acclimatisation with the modified rebreathing method. We assessed changes in steady state responses of middle cerebral artery velocity (MCAv), cerebrovascular conductance index (CVCi) and ventilation (VE) to varied levels of CO2 in 21 lowlanders (9 females; 21 ± 1 years), at sea-level (SL), during initial exposure to 5,260m (ALT1), after 16 days of acclimatisation (ALT16) and upon re-exposure to altitude following either 7 (POST7) or 21 days (POST21) at low altitude (1,525m). In the non-acclimatised state (ALT1), MCAv and VE responses to CO2 were elevated compared to SL (by 79±75% and 14.8±12.3 L/min, respectively, P=0.004 & P=0.011). Acclimatisation at ALT16 further elevated both MCAv and VE responses to CO2 compared to ALT1 (by 89±70% and 48.3±32.0 L/min, respectively, P<0.001). The acclimatisation gained for VE responses to CO2 at ALT16 was retained by 38% upon re-exposure to altitude at POST7 (P=0.004 vs. ALT1), while no retention was observed for the MCAv responses (P>0.05). We found good agreement between steady-state and modified rebreathing estimates of MCAv and VE responses to CO2 across all three time points (P<0.001, pooled data). Regardless of the method of assessment, altitude acclimatisation elevates both the cerebrovascular and ventilatory responsiveness to CO2. Our data demonstrates that this enhanced ventilatory CO2 response is partly retained after 7 days at low altitude.